Model

<!ENTITY % option-model 
                        "((%id.class;)*,
                          label?, title?, subtitle*, alt-title*,
                          ( (%sec-level;)+ |
                            ( (%answer-para-level;)+, 
                              (%sec-level;)* ) ),
                          (%sec-back-matter-mix;)*,
                           explanation*)"                           >

Expanded Model

((object-id)*, label?, title?, subtitle*, alt-title*, ( (sec)+ | ( (address | alternatives | answer | answer-set | array | block-alternatives | boxed-text | chem-struct-wrap | code | fig | fig-group | graphic | media | preformat | question | question-wrap | question-wrap-group | supplementary-material | table-wrap | table-wrap-group | disp-formula | disp-formula-group | def-list | list | tex-math | mml:math | p | related-article | related-object | disp-quote | speech | statement | verse-group)+, (sec)* ) ), (fn-group | glossary | ref-list)*, explanation*)